The Key to the Universe???

The external tank (ET) amasses 30 to 33 tons and would make an excellent spaceship hull, propellant depot tank, space station module or surface habitat.  It could be orbited rather than discarded.  How many thousands of ETs and space station modules could we orbit with HLV cargo rockets if tourists were flying  to LEO routinely in methane/LOX fueled VTOLs?

NASA

The Shuttle main engine, above, is one of the best rocket engines ever built.  It has never failed after more than 100 flights.  It is kind of a high maintainence item, however.  The NERVA engine, below, could be attached to ETs converted into spaceships to propell them through interplanetary space.